1. Field of the Invention
The present invention relates to an epitaxial substrate and a semiconductor element.
2. Description of the Background Art
Y. Irokawa, et. al., Appl. Phys. Lett., Vol. 83, No. 11, 15 Sep. 2003 describes a PIN diode. The PIN diode is equipped with an epitaxial layer grown on a GaN free-standing substrate. The thick film used as the GaN free standing substrate is grown using the hydride vapor phase epitaxy (HVPE) method on an A12O3 substrate. A laser is applied to this thick film to separate it from the A12O3 substrate to form the GaN free-standing substrate. On this GaN free-standing substrate, metal-organic vapor phase growth epitaxy is used to grow an undoped gallium nitride film with a thickness of 3 microns. Next, an Mg-doped gallium nitride film having a thickness of 0.3 microns is grown on this undoped gallium nitride film. The GaN free-standing substrate, the undoped gallium nitride film, and the Mg-doped gallium nitride film form a PIN structure.
In P. Kozodoy, et al., Appl. Phys. Lett., Vol. 73, No. 7, 17 Aug. 1998, characteristics of a gallium nitride pn junction are described. First, a GaN film having a thickness of 2 macrons is grown on a c-plane sapphire substrate using metal-organic vapor phase growth epitaxy with an SiO2 mask for LEO recombination. The mask is formed as stripes with 5 micron openings and spaced at intervals of 45 microns. In LEO growth, gallium nitride grows perpendicular to the mask openings and overgrows horizontally on the mask. The height and the overgrowth length of the grown gallium nitride are both approximately 8 microns. A pn junction diode is formed on this LEO gallium nitride region. This pn junction diode includes an n-type GaN film having a thickness of 1 micron, a magnesium-doped p-type GaN film having a thickness of 0.5 microns grown on top of this. The size of this pn junction diode is 2 microns×20 microns.
In the gallium nitride pn junction diode described in the Kozodoy paper, reverse leakage current at low-dislocation areas (less than 106 cm−2) is reduced compared to high-dislocation areas (approximately 4×108 cm−2), indicating that reverse breakdown is improved. However, the device structure in this paper is complex and production of the device on the low-dislocation area is not practical. In the Irokawa paper described above, the thickness of the GaN epitaxial layer is 3 microns and is not sufficient for a carrier concentration of 5×1016 cm−3. The reverse blocking voltage of the PIN diode in the Irokawa paper is also not high enough.
The breakdown mechanism of nitride semiconductors such as diodes is as follows. When the field intensity at the Schottky junction or the PN junction, which is the maximum field intensity in the reverse bias state, exceeds a critical value, impact ionization causes reverse leakage current to increase suddenly. This is the phenomenon known as breakdown. The ideal breakdown is when the epitaxial layer is thick enough where the depletion layer extends and the depletion layer is in the epitaxial layer even when the field intensity at the junction reaches the critical value. However, if the thickness of the epitaxial layer is not adequate for the carrier concentration, causing depletion of the entire thickness of the epitaxial layer before the field intensity at the junction reaches the critical value (punch-through), the field intensity at the junction will reach the critical value earlier so that breakdown takes place at a lower voltage compared to the ideal case described above. Also, since the depletion layer extends to the boundary surface between the epitaxial layer and the substrate, leaked current caused by imperfections in the boundary surface can reduce the reverse characteristics of the leakage current, possibly lowering the breakdown voltage. If punch-through takes place due to these factors, the breakdown voltage will be lower.
The object of the present invention is to overcome these problems and to provide a semiconductor element that contains a group III compound semiconductor layer that includes a structure for improving breakdown. Another object of the present invention is to provide an epitaxial substrate for this semiconductor element.